1. Field of the Invention
The present invention relates to electronic devices, and in particular, to removing and reworking underfilled integrated circuit chips on an electronic module.
2. Description of Related Art
Electronic components include both Single Chip Modules (SCMs) and Multi Chip Modules (MCMs). While SCMs contain a single chip, MCMs contain numerous chips on a substrate, and as such, are key components for many high-end computer servers and mainframes.
Often, the electronic components in SCMs and MCMs are joined to other components by solder interconnections whereby these solder interconnections are made by soldering pads on a surface of a first of these electronic components to corresponding soldering pads on the surface of the second component. Typical solder surface mount processes involve screening solder paste onto exposed metallic pads of a board or substrate, followed by a thermal reflow to bring the solder into spherical shapes. Alternately, solder preforms may be attached to exposed metallic pads. The reflowed solder or solder ball perform is aligned to corresponding pads on another component, and then the entire assembly is reflowed to melt the solder and create a solder bond between the first and second components. This solder interconnection may be in the form of a ball grid array (BGA) or column grid array (CGA).
Flip chip joining, or controlled chip collapse connection (C4) technology, also exploits the use of a relatively small solder bump to join the pads on the chip to corresponding pads on the substrate. Electrical and mechanical interconnects are formed simultaneously by reflowing the bumps at an elevated temperature above the melting point of the solder. The C4 joining process is self-aligning in that the wetting action of the solder will align the chip's bump pattern to the corresponding substrate pads. These solder bumps are then underfilled (or encapsulated) to protect them from moisture, mobile ions, corrosives (e.g., acidic or alkaline species) and separation of the solder bump electrical connections, all of which will adversely affect the performance of the package. The underfill also provides mechanical support to the chip, and maintains both the mechanical integrity of the chip due to stresses from assembly operations and the overall electrical reliability of the solder bumps and electronic package.
After forming the solder bumps between the chip(s) and substrate, testing one or more of these chips may show them to be defective. Without underfill present, the defective chip(s) can be replaced with new chips. The removal of a chip is usually performed by heating and lifting the defective chip from the substrate, followed by replacing it with a new chip. This is termed rework and can often be performed numerous times without degrading the quality or reliability of the reworked electronic module. This rework provides great monetary value in recovery of expensive modules.
Sometimes a defective die is not discovered before underfill introduction and during routine testing. This can occur due to incomplete initial test coverage or the inability to drive all circuits under highly stressful thermal environments, which are difficult to create without underfilling the die. When a module with underfill residing between the chip and substrate is found to have a defective die this presents a number of problems in the rework process since the underfill material is generally a very durable material by design. For instance, wherein the solder bumps are lead free solder interconnects, the lead-free interconnections often require the use of higher temperatures during reflow attachment, and even higher temperatures for rework processing. However, the higher rework temperatures can irreparably damage adjacent components on an organic or ceramic board, and as such, rework of lead free alloy containing assemblies has become a critical issue in the qualification of this technology. Thus, rework processes for various assembly materials must be selective for a particular material, such that, it causes no detriment to the substrate integrity and electrical performance. The removal process must not damage the board onto which the chips are mounted and is preferably simple and accurate in its ability to selectively rework what are often very tightly packed components. The resulting joins between the reworked chip and board must be reliable to recreate the overall integrity of the module. The chip must also be capable of being tested and underfilled again to provide the same mechanical integrity that was present in the initial chip join. It is also required that the removal method be environmentally and chemically suitable for use in a manufacturing environment.
Therefore, a need continues to exist in the art for providing improved methods and structures for the rework of electronic modules, and in particular, for the removal of electronic components joined to organic boards by solder interconnections for their subsequent use and re-use in electronic assemblies and systems.